Laser processing method

ABSTRACT

A laser processing method which can highly accurately cut objects to be processed having various laminate structures is provided. An object to be processed comprising a substrate and a laminate part disposed on the front face of the substrate is irradiated with laser light L while a light-converging point P is positioned at least within the substrate, so as to form a modified region due to multiphoton absorption at least within the substrate, and cause the modified region to form a starting point region for cutting. When the object is cut along the starting point region for cutting, the object  1  can be cut with a high accuracy.

TECHNICAL FIELD

The present invention relates to a laser processing method used forcutting an object to be processed which is constructed such that asurface of a substrate is provided with a laminate part.

BACKGROUND ART

Recently, techniques for highly accurately cutting objects to beprocessed having various laminate structures such as one in which asemiconductor active layer of GaN or the like is grown as a crystal onan Al₂O₃ substrate for a semiconductor device, one in which a glasssubstrate is attached onto another glass substrate for a liquid crystaldisplay unit, etc. have been in demand.

In general, blade dicing and diamond scribing have conventionally beenemployed for cutting the objects to be processed having these laminatestructures.

The blade dicing is a method in which an object to be processed isshaven and cut with a diamond blade or the like. On the other hand, thediamond scribing is a method in which the front face of an object to beprocessed is provided with a scribe line by a diamond-point tool, and aknife edge is pressed against the rear face of the object along thescribe line, so as to divide and cut the object.

DISCLOSURE OF THE INVENTION

However, when the object to be processed is the above-mentioned one fora liquid crystal display unit, for example, a gap is provided betweenthe glass substrates, where cutting dust and lubricating/washing watermay enter in the blade dicing.

In the diamond scribing, not only the front face but also the rear faceof the object to be processed must be provided with a scribe line incases where the object has a substrate with a high degree of hardnesssuch as an Al₂O₃ substrate, where the object is one in which glasssubstrates are attached to each other, etc., whereby cutting failuresmay occur because of positional deviations between the scribe linesprovided in the front and rear faces.

In view of such circumstances, it is an object of the present inventionto provide a laser processing method which can solve the problemsmentioned above and cut, with a high accuracy, an object to be processedhaving various laminate structures.

For achieving the above-mentioned object, the present invention providesa laser processing method comprising the step of irradiating an objectto be processed comprising a substrate and a laminate part disposed on afront face of the substrate with laser light while positioning alight-converging point at least within the substrate, so as to form amodified region due to multiphoton absorption at least within thesubstrate, and causing the modified region to form a starting pointregion for cutting along a line along which the object should be cut inthe object inside by a predetermined distance from a laser lightincident face of the object.

In this laser processing method, a starting point region for cuttingalong a desirable line along which the object should be cut for cuttingthe object to be processed can be formed within the substrate in theobject by the modified region formed by a phenomenon of multiphotonabsorption. Also, in view of the thickness, material, etc. of thelaminate part disposed on the front face of the substrate, the distancefrom the front face of the substrate to the modified region in thestarting point region for cutting can be controlled by regulating theposition at which the light-converging point of the laser light isplaced. Therefore, the object to be processed constructed such that thelaminate part is disposed on the front face of the substrate can bebroken and cut with a relatively small force, whereby objects to beprocessed having various laminate structures can be cut with a highaccuracy.

Here, the laminate part disposed on the front face of the substraterefers to one deposited on the front face of the substrate, one attachedto the front face of the substrate, etc., regardless of whether itsmaterial is different from or identical to that of the substrate. Thelaminate part disposed on the front face of the substrate includes onedisposed in close contact with the substrate, one disposed with a gapfrom the substrate, etc. Examples of the laminate part includesemiconductor active layers formed by crystal growth on the substrate,glass substrates attached onto other glass substrates, etc. The laminatepart also includes one in which a plurality of layers are formed frommaterials different from each other. The expression “within thesubstrate” encompasses the front face of the substrate provided with thelaminate part as well. The light-converging point refers to a locationat which laser light is converged. The starting point region for cuttingrefers to a region to become a start point for cutting when the objectto be processed is cut. Therefore, the starting point region for cuttingis a part to cut where cutting is to be performed in the object. Thestarting point region for cutting may be produced by continuouslyforming a modified region or intermittently forming a modified region.

In another aspect, the present invention provides a laser processingmethod comprising the step of irradiating an object to be processedcomprising a substrate and a laminate part disposed on a front face ofthe substrate with laser light while positioning a light-convergingpoint at least within the substrate under a condition with a peak powerdensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less, so as to form a modified region including acrack region at least within the substrate, and causing the modifiedregion to form a starting point region for cutting along a line alongwhich the object should be cut in the object inside by a predetermineddistance from a laser light incident face of the object.

When the substrate is irradiated with laser light under this condition,a phenomenon of optical damage due to multiphoton absorption occurswithin the substrate. This optical damage induces thermal distortionwithin the substrate, thereby forming a crack region within thesubstrate. The crack region is an example of the above-mentionedmodified region. An example of the substrate subjected to this laserprocessing method is a member including glass.

In still another aspect, the present invention provides a laserprocessing method comprising the step of irradiating an object to beprocessed comprising a substrate and a laminate part disposed on a frontface of the substrate with laser light while positioning alight-converging point at least within the substrate under a conditionwith a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 μs or less, so as to forma modified region including a molten processed region at least withinthe substrate, and causing the modified region to form a starting pointregion for cutting along a line along which the object should be cut inthe object inside by a predetermined distance from a laser lightincident face of the object.

When the substrate is irradiated with laser light under this condition,the inside of the object is locally heated by multiphoton absorption.This heating forms a molten processed region within the object. Themolten processed region is an example of the above-mentioned modifiedregion. An example of the object to be processed by this laserprocessing method is a member including a semiconductor material.

In still another aspect, the present invention provides a laserprocessing method comprising the step of irradiating an object to beprocessed comprising a substrate and a laminate part disposed on a frontface of the substrate with laser light while positioning alight-converging point at least within the substrate under a conditionwith a peak power density of at least 1×10⁸ (W/am²) at thelight-converging point and a pulse width of 1 ns or less, so as to forma modified region including a refractive index change region which is aregion with a changed refractive index at least within the substrate,and causing the modified region to form a starting point region forcutting along a line along which the object should be cut in the objectinside by a predetermined distance from a laser light incident face ofthe object.

When the substrate is irradiated with laser light under this condition,multiphoton absorption occurs within the substrate. However, since thepulse width is very short, the energy caused by multiphoton absorptionis not transformed into thermal energy, so that a permanent structuralchange such as ionic valence change, crystallization, or polarizationorientation is induced within the substrate, whereby a refractive indexchange region is formed. The refractive index change region is anexample of the modified region. An example of the substrate subjected tothis laser processing method is a member including glass.

In still another aspect, the present invention provides a laserprocessing method comprising the step of irradiating an object to beprocessed comprising a substrate and a laminate part disposed on a frontface of the substrate with laser light while positioning alight-converging point at least within the substrate, so as to form amodified region at least within the substrate, and causing the modifiedregion to form a starting point region for cutting along a line alongwhich the object should be cut in the object inside by a predetermineddistance from a laser light incident face of the object. The modifiedregion may include at least one of a crack region which is a regionwhere a crack is generated within the substrate, a molten processedregion which is a region subjected to melting within the substrate, anda refractive index change region which is a region with a changedrefractive index within the substrate.

This laser processing method can highly accurately cut objects to beprocessed having various laminate structures because of reasons similarto those of the above-mentioned laser processing methods in accordancewith the present invention. The modified region may be formed because ofmultiphoton absorption or other reasons.

In still another aspect, the present invention provides a laserprocessing method comprising the step of irradiating an object to beprocessed comprising a substrate and a laminate part disposed on a frontface of the substrate with laser light while positioning alight-converging point within the substrate, irradiating the object withlaser light while positioning a light-converging point within thelaminate part, so as to form respective modified regions within thesubstrate and laminate part, and causing the modified regions to form astarting point region for cutting along a line along which the objectshould be cut in the object inside by a predetermined distance from alaser light incident face of the object. The modified region may includeat least one of a crack region which is a region where a crack isgenerated within the substrate, a molten processed region which is aregion subjected to melting within the substrate, and a refractive indexchange region which is a region with a changed refractive index withinthe substrate.

This laser processing method forms a starting point region for cuttingalong a line along which the object should be cut not only within thesubstrate but also within the laminate part, so that the object to beprocessed can be cut with a smaller force, whereby objects havingvarious laminate structures can be cut with a high accuracy. The formingof the modified region within the substrate and the forming of themodified region within the laminate part may be effected simultaneouslywhile using respective laser light sources different from each other orseparately (in any order) while using the same laser light source, forexample. The modified region may be formed because of multiphotonabsorption or other reasons.

In still another aspect, the present invention provides a laserprocessing method comprising the step of irradiating an object to beprocessed comprising a substrate and a laminate part disposed on a frontface of the substrate with laser light while positioning alight-converging point at least within the substrate, so as to form amodified region along a line along which the object should be cut atleast within the substrate, thereby cutting the object. The modifiedregion may include at least one of a crack region which is a regionwhere a crack is generated within the substrate, a molten processedregion which is a region subjected to melting within the substrate, anda refractive index change region which is a region with a changedrefractive index within the substrate.

In this laser processing method, a fracture naturally grows in thesubstrate and laminate part from the modified region formed within thesubstrate acting as a start point so as to extend along the line alongwhich the object should be cut, whereby the object can be cut. Thislaser processing method is effective when the laminate part is thinnerthan the substrate, for example. The modified region may be formedbecause of multiphoton absorption or other reasons.

Preferably, in the above-mentioned laser processing methods inaccordance with the present invention, the laser light irradiating thesubstrate while positioning the light-converging point therewithinirradiates the substrate from the rear face side of the substrate. Inthis case, even when the laminate part disposed on the front face of thesubstrate has a light-shielding or absorbing characteristic for laserlight, the modified region can form a starting point region for cuttingwithin the substrate of the object.

For achieving the above-mentioned object, in still another aspect, thepresent invention provides a laser processing method comprising thesteps of irradiating a substrate with laser light while positioning alight-converging point within the substrate, so as to form a modifiedregion due to multiphoton absorption within the substrate, and causingthe modified region to form a starting point region for cutting along aline along which the object should be cut in the object inside by apredetermined distance from a laser light incident face of thesubstrate; and providing a front face of the substrate with a laminatepart after the step of forming the starting point region for cutting.

In this laser processing method, a starting point region for cutting isformed within a substrate before a laminate part is disposed on thefront face of the substrate. Since the modified region is formed onlylocally by multiphoton absorption, the laser light is hardly absorbed bythe front face of the substrate, whereby the front face of the substratedoes not melt. Therefore, as in the case where no modified region isformed within the substrate, the front face of the substrate can beprovided with the laminate part, so as to form the object to beprocessed. Because of the same reason as that mentioned above, thusformed object to be processed can be broken and cut with a relativelysmall force from the starting point region for cutting formed within thesubstrate as a start point, whereby objects to be processed havingvarious laminate structures can be cut with a high accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an object to be processed during laserprocessing in the laser processing method in accordance with anembodiment of the present invention;

FIG. 2 is a sectional view of the object to be processed taken along theline II-II of FIG. 1;

FIG. 3 is a plan view of the object to be processed after laserprocessing by the laser processing method in accordance with theembodiment;

FIG. 4 is a sectional view of the object to be processed taken along theline IV-IV of FIG. 3;

FIG. 5 is a sectional view of the object to be processed taken along theline V-V of FIG. 3;

FIG. 6 is a plan view of the object to be processed cut by the laserprocessing method in accordance with the embodiment;

FIG. 7 is a graph showing relationships between the electric fieldintensity and crack spot size in the laser processing method inaccordance with the embodiment;

FIG. 8 is a sectional view of the object to be processed in a first stepof the laser processing method in accordance with the embodiment;

FIG. 9 is a sectional view of the object to be processed in a secondstep of the laser processing method in accordance with the embodiment;

FIG. 10 is a sectional view of the object to be processed in a thirdstep of the laser processing method in accordance with the embodiment;

FIG. 11 is a sectional view of the object to be processed in a fourthstep of the laser processing method in accordance with the embodiment;

FIG. 12 is a view showing a photograph of a cut section in a part of asilicon wafer cut by the laser processing method in accordance with theembodiment;

FIG. 13 is a graph showing relationships between the laser lightwavelength and the internal transmittance of a silicon substrate in thelaser processing method in accordance with the embodiment;

FIG. 14 is a schematic diagram of the laser processing apparatus inaccordance with the embodiment;

FIG. 15 is a flowchart for explaining the laser processing method inaccordance with the embodiment;

FIG. 16A is a view showing a case where a modified region is formed nearthe rear face of the substrate in the object to be processed inaccordance with Example 1;

FIG. 16B is a view showing a case where a modified region is formed nearthe front face of the substrate in the object to be processed inaccordance with Example 1;

FIG. 17A is a view showing a case where a modified region is formed nearthe rear face of the substrate in the object to be processed inaccordance with Example 2;

FIG. 17B is a view showing a case where a modified region is formed nearthe front face of the substrate in the object to be processed inaccordance with Example 2;

FIG. 18A is a view showing a case where modified regions are formed nearthe front face of the substrate and in the laminate part in the objectto be processed in accordance with Example 3;

FIG. 18B is a view showing a case where a modified region is formed nearthe rear face of the substrate in the object to be processed inaccordance with Example 3;

FIG. 18C is a view showing a case where a modified region is formed nearthe front face of the substrate in the object to be processed inaccordance with Example 3;

FIG. 19 is a view showing the object to be processed in accordance withExample 4;

FIG. 20A is a view showing a case where respective modified regions areformed near the front face of the substrate and near the front face ofthe laminate part in the object to be processed in accordance withExample 5;

FIG. 20B is a view showing a case where respective modified regions areformed near the rear face of the substrate and near the rear face of thelaminate part in the object to be processed in accordance with Example5;

FIG. 21A is a view showing a case where respective modified regions areformed near the front face of the substrate and near the rear face ofthe laminate part in the object to be processed in accordance withExample 5;

FIG. 21B is a view showing a case where respective modified regions areformed near the rear face of the substrate and near the front face ofthe laminate part in the object to be processed in accordance withExample 5; and

FIG. 22 is an enlarged view showing a major part of the object to beprocessed in accordance with Example 6.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, a preferred embodiment of the present invention willbe explained in detail with reference to drawings. In the laserprocessing method in accordance with this embodiment, a modified regiondue to multiphoton absorption is formed within an object to beprocessed. Hence, the laser processing method, multiphoton absorption inparticular, will be explained at first.

A material becomes optically transparent if its absorption bandgap E_(G)is greater than a photon energy hν. Hence, the condition under whichabsorption occurs in the material is hν>E_(G). However, even whenoptically transparent, the material yields absorption under thecondition of nhν>E_(G) (n=2, 3, 4, . . . ) if the intensity of laserlight is very high. This phenomenon is known as multiphoton absorption.In the case of pulse waves, the intensity of laser light is determinedby the peak power density (W/cm²) of laser light at a light-convergingpoint thereof. The multiphoton absorption occurs, for example, at a peakpower density (W/Cm²) of 1×10⁸ (W/cm²) or higher. The peak power densityis determined by (energy per pulse of laser light at thelight-converging point)/(laser light beam spot cross-sectionalarea×pulse width) In the case of a continuous wave, the intensity oflaser light is determined by the electric field strength (W/cm²) oflaser light at the light-converging point.

The principle of laser processing in accordance with the embodimentutilizing such multiphoton absorption will now be explained withreference to FIGS. 1 to 6. FIG. 1 is a plan view of an object to beprocessed 1 during laser processing; FIG. 2 is a sectional view of theobject 1 taken along the line II-II of FIG. 1; FIG. 3 is a plan view ofthe object 1 after laser processing; FIG. 4 is a sectional view of theobject 1 taken along the line IV-IV of FIG. 3; FIG. 5 is a sectionalview of the object 1 taken along the line V-V of FIG. 3; and FIG. 6 is aplan view of the cut object 1.

As shown in FIGS. 1 and 2, the front face 3 of the object 1 has adesirable line along which the object should be cut 5 for cutting theobject 1. The line along which the object should be cut 5 is a linearlyextending virtual line (the object 1 may also be formed with an actualline acting as the line along which the object should be cut 5). In thelaser processing in accordance with this embodiment, the object 1 isirradiated with laser light L such that a light-converging point P ispositioned within the object 1 under a condition causing multiphotonabsorption, so as to form a modified region 7. Here, thelight-converging point is a location where the laser light L isconverged.

The laser light L is relatively moved along the line along which theobject should be cut 5 (in the direction of arrow A), so as to move thelight-converging point P along the line along which the object should becut 5. This forms the modified region 7 along the line along which theobject should be cut 5 only within the object 1 as shown in FIGS. 3 to5, and the modified region 7 forms a starting point region for cutting(part to cut) 8. In the laser processing method in accordance with thisembodiment, no modified region 7 is formed upon heating the object 1 bycausing the object 1 to absorb the laser light L. Instead, the laserlight L is transmitted through the object 1, so as to generatemultiphoton absorption within the object 1, thereby forming the modifiedregion 7. Hence, the laser light L is hardly absorbed by the front face3 of the object 1, whereby the front face 3 of the object 1 does notmelt.

If a start point exists at a location to cut when cutting the object 1,the object 1 fractures from this start point and thus can be cut with arelatively small force as shown in FIG. 6. This makes it possible to cutthe object 1 without generating unnecessary fractures in the front face3 of the object 1.

There seem to be the following two ways of cutting the object from thestarting point region for cutting acting as a start point. The firstcase is where, after forming the starting point region for cutting, anartificial force is applied to the object, so that the object fracturesfrom the starting point region for cutting acting as a start point,whereby the object is cut. This is the cutting in the case where theobject has a large thickness, for example. The application of anartificial force encompasses application of bending stress and shearingstress along the starting point region for cutting of the object, andexertion of a temperature difference upon the object to generate thermalstress, for example. The other case is where a starting point region forcutting is formed, so that the object is naturally fractured in across-sectional direction (thickness direction) of the object from thestarting point region for cutting acting as a start point, whereby theobject is cut. This is enabled, for example, by forming the startingpoint region for cutting by a single row of modified regions when theobject has a small thickness, and by a plurality of rows of modifiedregions aligned in the thickness direction when the object has a largethickness. Even in the case of natural fracturing, fractures do notextend to the front face at a location not formed with the startingpoint region for cutting in the part to cut, whereby only the partcorresponding to the location formed with the starting point region forcutting can be fractured. Thus, fracturing can be regulated well. Such afracturing method with favorable controllability is quite effective,since objects to be processed such as silicon wafers have recently beenapt to become thinner.

The modified region formed by multiphoton absorption in this embodimentincludes the following cases (1) to (3):

(1) Case Where the Modified Region is a Crack Region Including One or aPlurality of Cracks

An object to be processed (e.g., glass or a piezoelectric material madeof LiTaO₃) is irradiated with laser light while a light-converging pointis positioned therewithin under a condition with an electric fieldintensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less. This pulse width is a condition under whicha crack region can be formed only within the object while generatingmultiphoton absorption without causing unnecessary damages to theobject. This generates a phenomenon of optical damage due to multiphotonabsorption within the object. This optical damage induces thermaldistortion within the object, thereby forming a crack regiontherewithin. The upper limit of electric field intensity is 1×10¹²(W/cm²), for example. The pulse width is preferably 1 ns to 200 ns, forexample. The forming of a crack region due to multiphoton absorption isdescribed, for example, in “Internal Marking of Glass Substrate bySolid-state Laser Harmonics”, Proceedings of 45th Laser MaterialsProcessing Conference (December 1998), pp. 23-28.

The inventors determined relationships between the electric fieldintensity and the magnitude of crack by an experiment. Conditions forthe experiment are as follows:

-   -   (A) Object to be processed: Pyrex (registered trademark) glass        (having a thickness of 700 μm)    -   (B) Laser        -   Light source: semiconductor laser pumping Nd:YAG laser        -   Wavelength: 1064 nm        -   Laser light spot cross-sectional area: 3.14×10⁻⁸ cm²        -   Oscillation mode: Q-switch pulse        -   Repetition frequency: 100 kHz        -   Pulse width: 30 ns        -   Output: output<1 mJ/pulse        -   Laser light quality: TEM₀₀        -   Polarization characteristic: linear polarization    -   (C) Light-converging lens        -   Transmittance with respect to laser light wavelength: 60%    -   (D) Moving speed of a mounting table mounting the object: 100        mm/sec

Here, the laser light quality being TEM₀₀ indicates that the lightconvergence is so high that, light can be converged up to about thewavelength of laser light.

FIG. 7 is a graph showing the results of the above-mentioned experiment.The abscissa indicates peak power density. Since laser light is pulselaser light, its electric field intensity is represented by the peakpower density. The ordinate indicates the size of a crack part (crackspot) formed within the object processed by one pulse of laser light.Crack spots gather, so as to form a crack region. The size of a crackspot refers to that of the part of dimensions of the crack spot yieldingthe maximum length. The data indicated by black circles in the graphrefers to a case where the light-converging lens (C) has a magnificationof ×100 and a numerical aperture (NA) of 0.80. On the other hand, thedata indicated by white circles in the graph refers to a case where thelight-converging lens (C) has a magnification of ×50 and a numericalaperture (NA) of 0.55. It is seen that crack spots begin to occur withinthe object when the peak power density reaches about 10¹¹ (W/cm²), andbecome greater as the peak power density increases.

A mechanism by which the object to be processed is cut upon formation ofa crack region in the laser processing in accordance with thisembodiment will now be explained with reference to FIGS. 8 to 11. Asshown in FIG. 8, the object 1 is irradiated with laser light L whilepositioning the light-converging point P within the object 1 under acondition where multiphoton absorption occurs, so as to form a crackregion 9 therewithin along a line along which the object should be cut.The crack region 9 is a region including one or a plurality of crackspots. The crack region 9 forms a starting point region for cutting. Asshown in FIG. 9, the crack further grows while using the crack region 9as a start point (i.e., using the starting point region for cutting as astart point) As shown in FIG. 10, the crack reaches the front face 3 andrear face 21 of the object 1. As shown in FIG. 11, the object 1 breaks,so as to be cut. The crack reaching the front face and rear face of theobject may grow naturally or grow as a force is applied to the object,

(2) Case Where the Modified Region is a Molten Processed Region

An object to be processed (e.g., a semiconductor material such assilicon) is irradiated with laser light while a light-converging pointis positioned therewithin under a condition with an electric fieldintensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less. As a consequence, the inside of the objectis locally heated by multiphoton absorption. This heating forms a moltenprocessed region within the object. The molten processed region refersto a region once melted and then re-solidified, a region just in amelted state, or a region in the process of re-solidifying from itsmelted state, and may also be defined as a phase-changed region or aregion having changed its crystal structure. The molten processed regionmay also be regarded as a region in which a certain structure haschanged into another structure in monocrystal, amorphous, andpolycrystal structures. Namely, it refers to a region in which amonocrystal structure has changed into an amorphous structure, a regionin which a monocrystal structure has changed into a polycrystalstructure, and a region in which a monocrystal structure has changedinto a structure including an amorphous structure and a polycrystalstructure, for example. When the object is a silicon monocrystalstructure, the molten processed region is an amorphous siliconstructure, for example. The upper limit of electric field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns to 200ns, for example.

By an experiment, the inventors have verified that a molten processedregion is formed within a silicon wafer. Conditions for the experimentare as follows:

-   -   (A) Object to be processed: silicon wafer (having a thickness of        350 μm and an outer diameter of 4 inches)    -   (B) Laser        -   Light source: semiconductor laser pumping Nd:YAG laser        -   Wavelength: 1064 nm        -   Laser light spot cross-sectional area: 3.14×10⁻⁸ cm²        -   Oscillation mode: Q-switch pulse        -   Repetition frequency: 100 kHz        -   Pulse width: 30 ns        -   Output: 20 μJ/pulse        -   Laser light quality: TEM₀₀        -   Polarization characteristic: linear polarization    -   (C) Light-converging lens        -   Magnification: ×50        -   N. A.: 0.55        -   Transmittance with respect to laser light wavelength: 60%    -   (D) Moving speed of a mounting table mounting the object: 100        mm/sec

FIG. 12 is a view showing a photograph of a cut section in a part of asilicon wafer cut by laser processing under the above-mentionedconditions. A molten processed region 13 is formed within a siliconwafer 11. The size of the molten processed region 13 formed under theabove-mentioned conditions is about 100 μm in the thickness direction.

The fact that the molten processed region 13 is formed by multiphotonabsorption will now be explained. FIG. 13 is a graph showingrelationships between the wavelength of laser light and thetransmittance within the silicon substrate. Here, respective reflectingcomponents on the front face side and rear face side of the siliconsubstrate are eliminated, whereby only the transmittance therewithin isrepresented. The above-mentioned relationships are shown in the caseswhere the thickness t of the silicon substrate is 50 μm, 100 μm, 200 μm,500 μm, and 1000 μm, respectively.

For example, it is seen that laser light is transmitted through thesilicon substrate by at least 80% at 1064 nm, where the wavelength ofNd:YAG laser is located, when the silicon substrate has a thickness of500 μm or less. Since the silicon wafer 11 shown in FIG. 12 has athickness of 350 μm, the molten processed region 13 due to multiphotonabsorption is formed near the center of the silicon wafer, i.e., at apart separated from the front face by 175 μm. The transmittance in thiscase is 90% or greater with reference to a silicon wafer having athickness of 200 μm, whereby the laser light is absorbed within thesilicon wafer 11 only slightly and is substantially transmittedtherethrough. This means that the molten processed region 13 is notformed by laser light absorption within the silicon wafer 11 (i.e., notformed upon usual heating with laser light), but by multiphotonabsorption. The forming of a molten processed region by multiphotonabsorption is described, for example, in “Processing CharacteristicEvaluation of Silicon by Picosecond Pulse Laser”, Preprints of theNational Meeting of Japan Welding Society, No. 66 (April 2000), pp.72-73.

Here, a fracture is generated in the cross-sectional direction whileusing a molten processed region as a start point, whereby the siliconwafer is cut when the fracture reaches the front face and rear face ofthe silicon wafer. The fracture reaching the front face and rear face ofthe silicon wafer may grow naturally or grow as a force is applied tothe silicon wafer. The fracture naturally grows from the starting pointregion for cutting to the front face and rear face of the silicon waferin any of the cases where the fracture grows from the molten processedregion in a melted state and where the fracture grows from the moltenprocessed region in the process of re-solidifying from the melted state.In any of these cases, the molten processed region is formed only withinthe silicon wafer. In the cut section after cutting, the moltenprocessed region is formed only therewithin as shown in FIG. 12. When amolten processed region is formed within the object, unnecessaryfractures deviating from a line along which the object should be cut arehard to occur at the time of fracturing, which makes it easier tocontrol the fracturing.

(3) Case Where the Modified Region is a Refractive Index Change Region

An object to be processed (e.g., glass) is irradiated with laser lightwhile a light-converging point is positioned therewithin under acondition with an electric field intensity of at least 1×10⁸ (W/cm²) atthe light-converging point and a pulse width of 1 ns or less. Whenmultiphoton absorption is generated within the object with a very shortpulse width, the energy caused by multiphoton absorption is nottransformed into thermal energy, so that a permanent structural changesuch as ionic valence change, crystallization, or polarizationorientation is induced within the object, whereby a refractive indexchange region is formed. The upper limit of electric field intensity is1×10¹² (W/cm²), for example. The pulse width is preferably 1 ns or less,more preferably 1 ps or less, for example. The forming of a refractiveindex change region by multiphoton absorption is described, for example,in “Formation of Photoinduced Structure within Glass by FemtosecondLaser Irradiation”, Proceedings of 42th Laser Materials ProcessingConference (November 1997), pp. 105-111.

The cases of (1) to (3) are explained as modified regions formed bymultiphoton absorption in the foregoing. When a starting point regionfor cutting is formed as follows in view of the crystal structure of awafer-like object to be processed, the cleavage property thereof, andthe like, the substrate can be cut with a smaller force and a higheraccuracy while using the starting point region for cutting as a startpoint.

Namely, in the case of a substrate made of a monocrystal semiconductorhaving a diamond structure such as silicon, the starting point regionfor cutting is preferably formed in a direction along the (111) plane(first cleavage plane) or (110) plane (second cleavage plane). In thecase of a substrate made of a III-V family compound semiconductor havinga zinc ore type structure such as GaAs, the starting point region forcutting is preferably formed in a direction along the (110) plane. Inthe case of a substrate having a hexagonal crystal structure such assapphire (Al₂O₃), a starting point region for cutting is preferablyformed in a direction along the (1120) plane (A plane) or (1100) plane(M plane) while using the (0001) plane (C plane) as a principal plane.

When the substrate is formed with an orientation flat along a directionto be formed with the starting point region for cutting (e.g., in adirection along the (111) plane in the monocrystal silicon substrate) ora direction orthogonal to the direction to be formed with the startingpoint region for cutting, the starting point region for cuttingextending along the direction to be formed with the starting pointregion for cutting can be formed in the substrate in an easy andaccurate manner with reference to the orientation flat.

A laser processing apparatus used in the above-mentioned laserprocessing method will now be explained with reference to FIG. 14. FIG.14 is a schematic diagram of the laser processing apparatus 100.

The laser processing apparatus 100 comprises a laser light source 101for generating laser light L; a laser light source controller 102 forcontrolling the laser light source 101 so as to regulate the output,pulse width, etc. of laser light L and the like; a dichroic mirror 103,arranged so as to change the orientation of the optical axis of laserlight L by 90°, having a function of reflecting the laser light L; alight-converging lens 105 for converging the laser light L reflected bythe dichroic mirror 103; a mounting table 107 for mounting an object tobe processed 1 irradiated with the laser light L converged by thelight-converging lens 105; an X-axis stage 109 for moving the mountingtable 107 in the X-axis direction; a Y-axis stage 111 for moving themounting table 107 in the Y-axis direction orthogonal to the X-axisdirection; a Z-axis stage 113 for moving the mounting table 107 in theZ-axis direction orthogonal to the X- and Y-axis directions; and a stagecontroller 115 for controlling the movement of these three stages 109,111, 113.

This movement of light-converging point P in X(Y)-axis direction iseffected by moving the object 1 in the X(Y)-axis direction by theX(Y)-axis stage 109 (111). The Z-axis direction is a directionorthogonal to the front face 3 of the object 1, and thus becomes thedirection of focal depth of laser light L incident on the object 1.Therefore, moving the Z-axis stage 113 in the Z-axis direction canposition the light-converging point P of laser light L within the object1. This can place the light-converging point P at a desirable positionsuch as the substrate, the laminate part on the substrate, or the likein the object 1 when the object 1 has a multilayer structure, forexample.

The laser light source 101 is an Nd:YAG laser generating pulse laserlight. Known as other kinds of laser usable as the laser light source101 include Nd:YVO₄ laser, Nd:YLF laser, and titanium sapphire laser.Though pulse laser light is used for processing the object 1 in thisembodiment, continuous wave laser light may be used as long as it cancause multiphoton absorption.

The laser processing apparatus 100 further comprises an observationlight source 117 for generating a visible light beam for irradiating theobject 1 mounted on the mounting table 107, and a visible light beamsplitter 119 disposed on the same optical axis as that of the dichroicmirror 103 and light-converging lens 105. The dichroic mirror 103 isdisposed between the beam splitter 119 and light-converging lens 105.The beam splitter 119 has a function of reflecting about a half of avisual light beam and transmitting the remaining half therethrough, andis arranged so as to change the orientation of the optical axis of thevisual light beam by 90°. About a half of the visible light beamgenerated from the observation light source 117 is reflected by the beamsplitter 119, and thus reflected visible light beam is transmittedthrough the dichroic mirror 103 and light-converging lens 105, so as toilluminate the front face 3 of the object 1 including the line alongwhich the object should be cut 5 and the like. When the object 1 ismounted on the mounting table 107 such that the rear face of the object1 faces the light-converging lens 105, the “front face” mentioned abovebecomes the “rear face” as a matter of course.

The laser processing apparatus 100 further comprises an image pickupdevice 121 and an imaging lens 123 which are disposed on the sameoptical axis as that of the beam splitter 119, dichroic mirror 103, andlight-converging lens 105. An example of the image pickup device 121 isa CCD camera. The reflected light of the visual light beam havingilluminated the front face 3 including the line along which the objectshould be cut 5 and the like is transmitted through the light-converginglens 105, dichroic mirror 103, and beam splitter 119 and forms an imageby way of the imaging lens 123, whereas thus formed image is captured bythe image pickup device 121, so as to yield imaging data.

The laser processing apparatus 100 further comprises an imaging dataprocessor 125 for inputting the imaging data outputted from the imagepickup device 121, an overall controller 127 for controlling the laserprocessing apparatus 100 as a whole, and a monitor 129. According to theimaging data, the imaging data processor 125 calculates focal point datafor positioning the focal point of the visible light generated from theobservation light source 117 onto the front face 3 of the object 1.According to the focal point data, the stage controller. 115 controlsthe movement of the Z-axis stage 113, so that the focal point of visiblelight is positioned on the front face 3 of the object. Hence, theimaging data processor 125 functions as an autofocus unit. Also,according to the imaging data, the imaging data processor 125 calculatesimage data such as an enlarged image of the front face 3. The image datais sent to the overall controller 127, subjected to various kinds ofprocessing therein, and then sent to the monitor 129. As a consequence,an enlarged image or the like is displayed on the monitor 129.

Data from the stage controller 115, image data from the imaging dataprocessor 125, and the like are fed into the overall controller 127.According to these data as well, the overall controller 127 regulatesthe laser light source controller 102, observation light source 117, andstage controller 115, thereby controlling the laser processing apparatus100 as a whole. Thus, the overall controller 127 functions as a computerunit.

The laser processing method in accordance with this embodiment will nowbe explained with reference to FIGS. 14 and 15. FIG. 15 is a flowchartfor explaining the step of forming a starting point region for cutting.In this embodiment, the object to be processed 1 comprises a substrateand a laminate part disposed on the front face of the substrate. Theobject 1 is mounted on the mounting table 107 of the laser processingapparatus 100 shown in FIG. 14 such that the rear face of the substratefaces the light-converging lens 105. Namely, the laser light Lirradiates the object 1 from the rear face side of the substratetherein.

First, light absorption characteristics of the substrate of the object 1are determined by a spectrophotometer or the like which is not depicted.According to the results of measurement, a laser light source 101generating laser light L having a wavelength to which the substrate ofthe object 1 is transparent or exhibits a low absorption is chosen(S101). Since the laser light L irradiates the substrate from the rearface side thereof, laser processing is not obstructed even when thelaminate part disposed on the substrate has a light-shielding orabsorbing characteristic for the laser light.

Subsequently, in view of the thickness and refractive index of theobject 1, the amount of movement of the object 1 in the Z-axis directionin the laser processing apparatus 100 is determined (S103). This is anamount of movement of the object 1 in the Z-axis direction withreference to the light-converging point P of laser light L positioned atthe rear face of the object 1 in order for the light-converging point Pof laser light L to be placed at a desirable position within thesubstrate in the object 1. This amount of movement is fed into theoverall controller 127.

The object 1 is mounted on the mounting table 107 of the laserprocessing apparatus 100 such that the rear face of the substrate facesthe light-converging lens 105. Subsequently, visible light is generatedfrom the observation light source 117, so as to illuminate the rear faceof the substrate of the object 1 (S105). The illuminated rear faceincluding the line along which the object should be cut 5 is captured bythe image pickup device 121. The line along which the object should becut 5 is a desirable virtual line for cutting the object 1. The imagingdata captured by the imaging device 121 is sent to the imaging dataprocessor 125. According to the imaging data, the imaging data processor125 calculates such focal point data that the focal point of visiblelight from the observation light source 117 is positioned at the rearface of the substrate of the object 1 (S107).

The focal point data is sent to the stage controller 115. According tothe focal point data, the stage controller 115 moves the Z-axis stage113 in the Z-axis direction (S109). As a consequence, the focal point ofvisible light from the observation light source 117 is positioned at therear face of the substrate of the object 1. According to the imagingdata, the imaging data processor 125 calculates enlarged image data ofthe rear face of the substrate of the object 1 including the line alongwhich the object should be cut 5. The enlarged image data is sent to themonitor 129 by way of the overall controller 127, whereby an enlargedimage of the line along which the object should be cut 5 and itsvicinity is displayed on the monitor 129.

Movement amount data determined in step S103 has been fed into theoverall controller 127 beforehand, and is sent to the stage controller115. According to the movement amount data, the stage controller 115causes the Z-axis stage 113 to move the object 1 in the Z-axis directionto a position where the light-converging point P of laser light L ispositioned within the substrate of the object 1 (S111).

Subsequently, laser light L is generated from the laser light source101, so as to irradiate the line along which the object should be cut 5in the rear face of the substrate of the object 1. Since thelight-converging point P of the laser light L is positioned within thesubstrate of the object 1, a modified region is formed only within thesubstrate of the object 1. Then, the X-axis stage 109 and Y-axis stage111 are moved along the line along which the object should be cut 5,such that the modified region formed along the line along which theobject should be cut 5 forms a starting point region for cutting withinthe object 1 along the line along which the object should be cut 5(S113).

As explained in the foregoing, the laser processing method in accordancewith this embodiment irradiates the object 1 with the laser light L fromthe rear face side of the substrate therein, whereby the modified regionformed by multiphoton absorption within the substrate can form astarting point region for cutting along a desirable line along which theobject should be cut 5 for cutting the object 1. In view of thethickness, material, etc. of the laminate part disposed on thesubstrate, the position of the modified region formed within thesubstrate is controlled by regulating the position where thelight-converging point P of the laser light L is placed. Therefore, theobject 1 constructed such that the laminate part is disposed on thefront face of the substrate can be broken and cut with a relativelysmall force from the starting point region for cutting formed within thesubstrate as a start point.

The laminate part of the object 1 may be irradiated with laser light Lhaving a wavelength to which the laminate part is transparent orexhibits a low absorption while the light-converging point P ispositioned within the laminate part, such that a starting point regionfor cutting along the line along which the object should be cut 5 isalso formed within the laminate part. In this case, the object 1 can bebroken and cut with a smaller force.

Examples of the laser processing method in accordance with thisembodiment will now be explained with reference to FIGS. 16 to 21.

EXAMPLE 1

FIG. 16A is a view showing a case where a modified region 7 is formednear the rear face of a substrate 15 in the object to be processed 1 inaccordance with Example 1, whereas FIG. 16B is a view showing a casewhere a modified region 7 is formed near the front face of the substrate15 in the object 1 in accordance with Example 1. Examples of the object1 shown in FIGS. 16A and 16B include one used for next-generationhigh-speed/low-power-consumption devices and one for next-generationdevices.

The substrate 15, first laminate part 17 a, and second laminate part 17b for a next-generation high-speed/low-power-consumption device are Si(500 μm), SiO₂ (1 μm), and Si (3 μm respectively. On the other hand, thesubstrate 15, first laminate part 17 a, and second laminate part 17 bfor a next-generation device are Si(500 μm), SrTiO₃ (several hundredμm), and GaAs (several hundred μm), respectively (where values inparentheses indicate thickness).

When the modified region 7 is positioned near the rear face 21 of theobject 1, a knife edge 23 is pressed against the front face 3 of theobject 1 along a starting point region for cutting formed by themodified region 7 as shown in FIG. 16A, so as to break and cut theobject 1. This is because a large tensile stress among bending stressesgenerated by pressing the knife edge 23 acts on the modified region 7,whereby the object can be cut with a relatively small force. When themodified region 7 is positioned near the front face 3 of the object 1,on the other hand, the knife edge 23 is pressed against the rear face 21of the object 1 as shown in FIG. 16B because of the same reason, so asto break and cut the object 1.

Here, “the modified region 7 is positioned near the rear face 21 of theobject 1” means that the modified region 7 constituting the startingpoint region for cutting is formed so as to shift from the centerposition of the object 1 in the thickness direction (half thicknessposition) toward the rear face 21. Namely, it refers to a case where thecenter position of the width of the modified region 7 in the thicknessdirection of the object 1 is shifted from the center position of theobject 1 in the thickness direction toward the rear face 21, and is notlimited to a case where the whole modified region 7 is positioned on therear face 21 side from the center position of the object 1 in thethickness direction. Similarly, “the modified region 7 is positionednear the front face 3 of the object 1” means that the modified region 7constituting the starting point region for cutting is formed so as toshift from the center position of the object 1 in the thicknessdirection (half thickness position) toward the front face 3. Theforegoing also holds for the position where the modified region 7 isformed with respect to the substrate 15.

EXAMPLE 2

FIG. 17A is a view showing a case where a modified region 7 is formednear the rear face of a substrate 15 in the object to be processed 1 inaccordance with Example 2, whereas FIG. 17B is a view showing a casewhere a modified region 7 is formed near the front face of the substrate15 in the object 1 in accordance with Example 2. The object 1 shown inFIGS. 17A and 17B is one for a blue LD/LED, examples of its substrate15/laminate part 17 include Al₂O₃ (500 μm)/a laminated functional film(several hundred nm) in which a plurality of layers of semiconductorcrystals such as GaN are formed, and Al₂O₃ (500 μm)/a laminatedfunctional film (several hundred nm) in which a plurality of layers suchas a ZnO layer are formed (where values in parentheses indicatethickness).

When the modified region 7 is positioned near the rear face 21 of theobject 1, the knife edge 23 is pressed against the front face 3 of theobject 1 as shown in FIG. 17A, so as to break and cut the object 1,because of the same reason as in the case of the object 1 in accordancewith Example 1. When the modified region 7 is positioned near the frontface 3 of the object 1, on the other hand, the knife edge 23 is pressedagainst the rear face 21 of the object 1 as shown in FIG. 17B, so as tobreak and cut the object 1.

EXAMPLE 3

FIG. 18A is a view showing a case where respective modified regions 7are formed near the front face of a substrate 15 and in a laminate part17 in the object to be processed 1 in accordance with Example 3; FIG.18B is a case where a modified region 7 is formed near the rear face ofthe substrate 15 in the object 1 in accordance with Example 3; and FIG.18C is a view showing a case where a modified region 7 is formed nearthe front face of the substrate 15 in the object 1 in accordance withExample 3. The object 1 shown in FIGS. 18A to 18C is one for an infraredlight detecting device, examples of its substrate 15/laminate part 17include Al₂O₃ (500 μm)/PbSe (10 μm), and Al₂O₃ (500 μm)/HgCdTe (10 μm)(where values in parentheses indicate thickness).

When the modified region 7 is positioned near the front face 3 of theobject 1 as shown in FIGS. 18A and 18C, the knife edge 23 is pressedagainst the rear face 21 of the object 1, so as to break and cut theobject 1, from the same reason as in the case of the object 1 inaccordance with Example 1. When the modified region 7 is positioned nearthe rear face 21 of the object 1, on the other hand, the knife edge 23is pressed against the front face 3 of the object 1 as shown in FIG.18B, so as to break and cut the object 1.

EXAMPLE 4

FIG. 19 is a view showing the object to be processed 1 in accordancewith Example 4. The object 1 shown in FIG. 19 is multilayer glass, inwhich two glass substrates as a first laminate part 17 a and a secondlaminate part 17 b are attached together and laminated on a glasssubstrate as a substrate 15. The modified region 7 in each glasssubstrate is formed on the rear face 21 side of the object 1. The knifeedge 23 is pressed against the front face 3 of the object 1 in this caseas well, so as to break and cut the object 1, because of the same reasonas in the case of the object 1 in accordance with Example 1. In the casewhere the laminate part has a large thickness or high degree of hardnessas such, the object 1 can be broken and cut with a smaller force if astarting point region for cutting is formed within the laminate part aswell.

EXAMPLE 5

FIGS. 20A to 21B are views showing the object to be processed 1 inaccordance with Example 5. FIG. 20A is a view showing a case whererespective modified regions 7 are formed near the front face of asubstrate 15 and near the front face of a laminate part 17 in the isobject 1 in accordance with Example 5, whereas FIG. 20B is a viewshowing a case where respective regions 7 are formed near the rear faceof the substrate 15 and near the rear face of the laminate part 17 inthe object 1 in accordance with Example 5. FIG. 21A is a view showing acase where respective modified regions 7 are formed near the front faceof the substrate 15 and near the rear face of the laminate part 17 inthe object 1 in accordance with Example 5, whereas FIG. 21B is a viewshowing a case where respective regions 7 are formed near the rear faceof the substrate 15 and near the front face of the laminate part 17 inthe object 1 in accordance with Example 5.

The object 1 shown in FIGS. 20A to 21B is one for a reflection typeliquid crystal display unit. The substrate 15 is a glass substrate(having a thickness of 1.8 mm and an outer diameter of 8 inches) formedwith a common electrode, whereas the laminate part 17 is an Si substrate(having a thickness of 500 μm and an outer diameter of 8 inches) formedwith TFT. The substrate 15 and laminate part 17 are bonded to each otherwith an adhesive 25 while forming a gap therebetween for receiving aliquid crystal.

In the case of FIGS. 20A and 20B, the object 1 is irradiated with laserlight from the rear face 21 side, so as to form a modified region 7within the laminate part 17, and then the object 1 is irradiated withthe laser light from the rear face 21 side, so as to form a modifiedregion 7 within the substrate 15. This is because the laser light has awavelength to which both the substrate 15 and laminate part 17 aretransparent or exhibit a low absorption. Because of the same reason asin the case of the object 1 in accordance with Example 1, the knife edge23 is pressed against the rear face 21 of the object 1 in the case ofFIG. 20A, so as to break and cut the object 1. In the case of FIG. 20B,on the other hand, the knife edge 23 is pressed against the front face 3of the object 1, so as to break and cut the object 1.

When respective starting point regions for cutting are formed in thesubstrate 15 and laminate part 17 by using laser light having awavelength to which both the substrate 15 and laminate part 17 aretransparent or exhibit a low absorption, an operation of reversing theobject 1 performed in a conventional diamond scribing method can besaved, whereby the object 1 can be prevented from being broken and soforth at the time of reversing operation. This can also preventpositional deviations from occurring between the respective startingpoint regions for cutting formed in the substrate 15 and laminate part17, whereby the object 1 can be cut with a high accuracy. Further,lubricating/washing water which is required in the conventional bladedicing is unnecessary, whereby there is no problem of thelubricating/washing water entering the gap between the substrate 15 andlaminate part 17.

In the case of FIGS. 21A and 21B, the object 1 is irradiated with laserlight from the rear face 21 side, so as to form a modified region 7within the substrate 15, and then is irradiated with the laser lightfrom the front face 3 side, so as to form a modified region 7 within thelaminate part 17. In the case of FIG. 21A, because of the same reason asin the case of the object 1 in accordance with Example 1, the knife edge23 is initially pressed against the rear face 21 of the object 1, so asto break and cut the substrate 15, and then the knife edge 23 is pressedagainst the front face 3 of the object 1, so as to break and cut thelaminate part 17. In the case of FIG. 21B, on the other hand, the knifeedge 23 is initially pressed against the front face 3 of the object 1,so as to break and cut the substrate 15, and then the knife edge 23 ispressed against the rear face 21 of the object 1, so as to break and cutthe laminate part 17.

EXAMPLE 6

FIG. 22 is an enlarged sectional view showing a major part of the objectto be processed 1 in accordance with Example 6. This object 1 is one inwhich a number of chip forming regions F are disposed on a substrate 15which is a silicon wafer, whereas a dicing line region D is formedbetween the chip forming regions F, F adjacent each other. FIG. 22 showsa cross section of a part where the chip forming region F and the dicingline region D are continuous with each other. A line along which theobject should be cut is set along the dicing line region D.

As shown in this drawing, an interlayer insulating film (laminate part)31 is formed on the substrate 15, whereas a metal wiring layer 32 isdisposed on the interlayer insulating film 31 at the chip forming regionF in the substrate 15. Further, an interlayer insulating film (laminatepart) 33 is formed so as to cover the interlayer insulating film 31 andmetal wiring layer 32 on the substrate 15, whereas a metal wiring layer34 is formed on the interlayer insulating film 33 in the chip formingregion F of the substrate 15. The substrate 15 and the metal wiringlayer 32 are electrically connected to each other with a plug 35penetrating through the interlayer insulating film 31. The metal wiringlayers 32 and 34 are electrically connected to each other with a plug 36penetrating through the interlayer insulating film 33.

Thus configured object 1 is irradiated with laser light while alight-converging point is positioned within the substrate 15, so as toform a modified region 7 within the substrate 15 along the dicing lineregion D (i.e., along the line along which the object should be cut),and cause the modified region 7 to form a starting point region forcutting. Then, the object 1 can be cut with a high accuracy when theknife edge 23 is pressed against the front face 3 or rear face 21 of theobject 1 along the starting point region for cutting.

The object 1 can be cut with a high accuracy even when the insulatingfilms 31, 32 made of SiO₂, SiN, or the like are formed as laminate partson the line along which the object should be cut in the substrate 15 aswith the object 1 in accordance with Example 6 in the foregoing.

Though the embodiment of the present invention is explained in detail inthe foregoing, the present invention is not limited to theabove-mentioned embodiment as a matter of course.

Though the above-mentioned embodiment relates to a case where an objectto be processed comprising a substrate and a laminate part disposed onthe front face of the substrate is irradiated with laser light, so as toform a starting point region for cutting, a substrate may initially beirradiated with laser light, so as to form a starting point region forcutting, and then the front face of the substrate may be provided with alaminate part, so as to form an object to be processed in the presentinvention.

Though the starting point region for cutting is formed within thesubstrate before providing the surface of the substrate with a laminatepart in this laser processing method, the modified region is formed onlylocally by multiphoton absorption, so that the laser light is hardlyabsorbed by the front face of the substrate, whereby the front face ofthe substrate does not melt. Therefore, as in the case where no modifiedregion is formed within the substrate, the front face of the substratecan be provided with the laminate part, so as to form the object to beprocessed. Because of the same reason as in the above-mentionedembodiment, thus formed object to be processed can be broken and cutwith a relatively small force from the starting point region for cuttingformed within the substrate as a start point.

INDUSTRIAL APPLICABILITY

In the laser processing method in accordance with the present invention,as explained in the foregoing, a modified region formed by a phenomenonof multiphoton absorption can form a cut start area within a substrateof an object to be processed along a desirable line along which theobject should be cut for cutting the object. Also, in view of thethickness, material, etc. of the laminate part disposed on the frontface of the substrate, the distance from the front face of the substrateto the modified region in the starting point region for cutting can becontrolled by regulating the position where the light-converging pointof the laser light is positioned. Therefore, the object configured suchthat the laminate part is disposed on the front face of the substratecan be broken and cut with a relatively small force from the startingpoint region for cutting, formed within the substrate, acting as a startpoint. The laminate part may also be irradiated with laser light whilethe light-converging point is positioned therewithin. In this case, theobject can be broken and cut with a smaller force. Because of theforegoing, objects to be processed having various laminate structurescan be cut with a high accuracy.

1-13. (canceled)
 14. A laser processing method comprising the step ofirradiating an object to be processed comprising a substrate and alaminate part disposed on a front face of the substrate with laser lightwhile positioning a light-converging point at least within thesubstrate, so as to form a modified region due to multiphon absorptionat least within the substrate, and causing the modified region to form astarting point region for cutting along a line along which the objectshould be cut in the object inside by a predetermined distance from alaser light incident face of the object.
 15. A laser processing methodcomprising the step of irradiating an object to be processed comprisinga substrate and a laminate part disposed on a front face of thesubstrate with laser light while positioning a light-converging point atleast within the substrate under a condition with a peak power densityof at least 1×10⁸ (W/cm²) at the light-converging point and a pulsewidth of 1 μs or less, so as to form a modified region including a crackregion at least within the substrate, an causing the modified region toform a starting point region for cutting along a line along which theobject should be cut in the object inside by a predetermined distancefrom a laser light incident face of the object.
 16. A laser processingmethod comprising the step of irradiating an object to be processedcomprising a substrate and a laminate part disposed on a front face ofthe substrate with laser light while positioning a light-convergingpoint at least within the substrate under a condition with a peak powerdensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less, so as to form a modified region including amolten processed region at least within the substrate, and causing themodified region to form a starting point region for cutting along a linealong which the object should be cut in the object inside by apredetermined distance from a laser light incident face of the object.17. A laser processing method comprising the step of irradiating anobject to be processed comprising a substrate and a laminate partdisposed on a front face of the substrate with laser light whilepositioning a light-converging point at least within the substrate undera condition with a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 ns or less, so as to forma modified region including a refractive index change region which is aregion with a changed refractive index at least within the substrate,and causing the modified region to form a starting point region forcutting along a line along which the object should be cut in the objectinside by a predetermined distance from a laser light incident face ofthe object.
 18. A laser processing method comprising the step ofirradiating an object to be processed comprising a substrate and alaminate part disposed on a front face of the substrate with laser lightwhile positioning a light-converging point at least within thesubstrate, so as to form a modified region at least within thesubstrate, and causing the modified region to form a starting pointregion for cutting along a line along which the object should be cut inthe object inside by a predetermined distance from a laser lightincident face of the object.
 19. A laser processing method comprisingthe step of irradiating an object to be processed comprising a substrateand a laminate part disposed on a front face of the substrate with laserlight while positioning a light-converging point within the substrate,irradiating the object with laser light while positioning alight-converging point within the laminate part, so as to formrespective modified regions to form a starting point region for cuttingalong a line along which the object should be cut in the object insideby a predetermined distance from a laser light incident face of theobject.
 20. A laser processing method comprising the step of irradiatingan object to be processed comprising a substrate and a laminate partdisposed on a front face of the substrate with laser light whilepositioning a light-converging point at least within the substrate, soas to form a modified region along a line along which the object shouldbe cut at least within the substrate, thereby cutting the object.
 21. Alaser processing method according to one of claims 18-20, wherein themodified region includes at least one of a crack region which is aregion where a crack is generated within the substrate, a moltenprocessed region which is a region subjected to melting within thesubstrate, and a refractive index change region which is a region with achanged refractive index within the substrate.
 22. A laser processingmethod according to one of claims 14-20, wherein the laser lightirradiating the substrate while positioning the light-converging pointtherewithin irradiates the substrate from the rear face thereof.
 23. Alaser processing method comprising the steps of: irradiating a substratewith laser light while positioning a light-converging point within thesubstrate, so as to form a modified region due to multiphoton absorptionwithin the substrate, and causing the modified region to form a startingpoint region for cutting along a line along which the object should becut in the object inside by a predetermined distance from a laser lightincident face of the substrate; and providing a front face of thesubstrate with a laminate part after the step of forming the startingpoint region for cutting.
 24. A laser processing method comprising thestep of irradiating an object to be processed comprising a substratewhich is made of a semiconductor material and a laminate part disposedon a front face of the substrate with laser light while positioning alight-converging point at least with the substrate under a conditionwith a peak power density of at least 1×10⁸ (W/cm²) at thelight-converging point and a pulse width of 1 μs or less, so as to forma modified region at least within the substrate, and causing themodified region to form a starting point region for cutting along a linealong which the object should be cut in the object inside by apredetermined distance from a laser light incident face of the object.25. A laser processing method comprising the step of irradiating anobject to be processed comprising a substrate which is made of apiezoelectric material and a laminate part disposed on a front face ofthe substrate with laser light while positioning a light-convergingpoint at least within the substrate under a condition with a peak powerdensity of at least 1×10⁸ (W/cm²) at the light-converging point and apulse width of 1 μs or less, so as to form a modified region at leastwithin the substrate, and causing the modified region to form a startingpoint region for cutting along a line along which the object should becut in the object inside by a predetermined distance from a laser lightincident face of the object.
 26. A laser processing method comprisingthe step of irradiating an object to be processed comprising a substratewhich is made of a semiconductor material and a laminate part disposedon a front face of the substrate with laser light while positioning alight-converging point at least within the substrate, so as to form amolten processed region at least within the substrate, and causing themolten processed region to form a starting point region for cuttingalong a line along which the object should be cut in the object insideby a predetermined distance from a laser light incident face of theobject.